1. Field of the Invention
The present invention relates to a near-field exposure system which exposes a fine pattern on a photosensitive material such as a photoresist.
2. Description of the Related Art
The development in the photolithographic technology has been supported by the development in the reduction-projection exposure technology and the resist technology. In the reduction-projection exposure technology, performance is mainly determined by two fundamental quantities, the resolution RP and the focal depth DOP, which are respectively expressed as RP=k1xcex/NA and DOP=k2xcex/NA2, where xcex is an exposure wavelength of a projection optical system, NA is a numerical aperture of a projection lens, and k1 and k2 are coefficients. In order to increase the resolution in lithography, it is important to reduce the wavelength xcex and increase the numerical aperture NA of the projection lens. Although the resolution is improved with increase in the numerical aperture NA, the focal depth DOP is reduced in inverse proportion to the square of the numerical aperture NA. Therefore, currently, the reduction in the wavelength xcex is required for realizing fine lithography. Thus, the exposure light used in lithography has been changed from the g line having the wavelength of 436 nm to the i line having the wavelength of 365 nm, and currently use of the excimer laser light having the wavelength of 248 or 193 nm is becoming mainstream.
However, in the lithography using light, the resolution limit is the diffraction limit of the exposure light. Therefore, it is said that the finest line width obtained by use of the excimer laser light having the wavelength of 248 or 193 nm and a lens-series optical system is 100 nm. Further, in order to achieve a resolution of the nanometer order, it is necessary to use an electron-beam or X-ray lithography technique. In particular, the synchrotron orbital radiations (SORs) are used in the X-ray lithography.
The electron-beam lithography enables highly precise control for forming a pattern including nanometer-scale structures. In addition, the electron-beam lithography can achieve a considerably greater focal depth than the lithography using an optical system. Further, the electron-beam lithography enables direct drawing on a wafer without use of a mask. However, because of low throughput and high cost, the current electron-beam lithographic techniques are far from the level which enables mass production.
On the other hand, the X-ray lithography can achieve about one order of magnitude higher resolution and precision than the excimer laser exposure either when 1:1 projection exposure through a 1:1 mask is used, or when a reflective imaging X-ray optical system is used. However, it is not easy to realize the X-ray lithography since production of masks is not easy. In addition, equipment cost is high.
In order to solve the above problems, a method for exposing a photosensitive material such as a photoresist to near-field light has been proposed. In this method, exposure light is applied to an exposure mask in which a pattern of openings having dimensions smaller than the wavelength of the exposure light is formed, so that near-field light emerges through the openings of the exposure mask, and the photosensitive material is exposed to the near-field light. According to the above method, a fine pattern of the same order of magnitude as the openings in the exposure mask can be formed in the photosensitive material regardless of the wavelength of the exposure light. An example of an exposure system executing the above method is disclosed in Japanese Unexamined Patent Publication No. 2000-112116.
It is known that when a fine pattern in an exposure mask is transferred to a photosensitive material such as a photoresist by exposure to near-field light as above, and the fine pattern is constituted by straight lines extending in an identical direction as in the case of a diffraction grating, and the exposure light is linearly polarized in the same direction as the direction of the straight lines, thickening of straight lines formed on the photosensitive material can be prevented so that a finer pattern can be formed on the photosensitive material.
Based on the above knowledge, JUPP No. 2000-112116 discloses a near-field exposure mask in which an opening pattern and a grid polarizer are stacked on a mask substrate, where the opening pattern has dimensions smaller than the wavelength of exposure light, and the grid polarizer polarizes light in a direction parallel to the direction of the openings constituting the opening pattern.
However, when a polarizer is produced for each exposure mask as above, the exposure mask becomes expensive, and thus the cost of fine pattern exposure increases.
An object of the present invention is to provide a near-field exposure system which can expose a photosensitive material such as a photoresist to near-field light so as to form a fine pattern including thin linear portions, and suppress the cost of the exposure.
(1) According to the first aspect of the present invention, there is provided a near-field exposure system comprising: a light source which emits exposure light having a predetermined wavelength and being unpolarized; a polarizer plate which linearly polarizes the exposure light; an exposure mask which has a pattern of openings each having a dimension smaller than the wavelength of the exposure light, and is placed at such a position that the exposure light is applied to the exposure mask; an exposure table which holds a photosensitive material sensitive to the exposure light, at a position which near-field light emerging from the openings reaches; and a polarizer-plate holding means which holds the polarizer plate in such a manner that the polarizer plate can be moved between first and second positions, where the first position is in an optical path of the exposure light from the light source to the exposure mask, and the second position is outside the optical path.
The near-field exposure system according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv).
(i) The polarizer-plate holding means may hold the polarizer plate in such a manner that the polarizer plate can be rotated and directed to a direction in which polarizer plate linearly polarizes the exposure light.
(ii) The near-field exposure system the first aspect of the present invention may further comprise an indication means for indicating a direction in which the polarizer plate linearly polarizes the exposure light. For example, the indication means is an arrow indication provided on the polarizer plate.
(iii) The exposure light which is applied to the exposure mask may have a first component which is linearly polarized in a desired direction and a second component which is linearly polarized in a direction perpendicular to the desired direction, and a ratio of the second component to the first component may be 25% or smaller.
(iv) In the near-field exposure system having the above feature (iii), the ratio may be 15% or smaller.
(2) The advantages of the near-field exposure system according to the first aspect of the present invention are explained below.
The near-field exposure system according to the first aspect of the present invention comprises the polarizer-plate holding means, so that the polarizer plate which linearly polarizes the exposure light can be moved between the first position in the optical path of the exposure light from the light source to the exposure mask and the second position outside the optical path. That is, the polarizer plate can be selectively inserted in the optical path to the exposure mask.
In the case where the exposure mask has an opening pattern constituted by only lines extending in an identical direction, thickening of lines formed on the photosensitive material can be prevented when the exposure mask is placed in an optical path of the linearly polarized exposure light in such a manner that the direction of the lines constituting the opening pattern is identical to the direction of the linear polarization of the linearly polarized exposure light. That is, a fine pattern constituted by lines having small widths can be formed on the photosensitive material by the exposure when the exposure light is linearly polarized by insertion of the polarizer plate in the optical path, and coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization is achieved.
The coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization can be achieved, for example, by changing the direction of the exposure table on which the photosensitive material is held, or adjusting the direction of the photosensitive material held on the exposure table. Alternatively, in the case where the polarizer-plate holding means holds the polarizer plate in such a manner that the polarizer plate can be rotated, the direction of the linear polarization can be easily adjusted to the direction of the lines constituting the opening pattern in the exposure mask by rotating the polarizer plate.
In addition, in the case where the indication means for indicating the direction in which the polarizer plate linearly polarizes the exposure light is provided, the user can visually confirm the direction of the linear polarization in the operation for achieving the coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization. Therefore, the operation for achieving the coincidence between the above directions becomes further easier.
Further, investigations made by the present inventor have revealed that a pattern formed on a photosensitive material by exposure to linearly polarized exposure light is likely to be deformed to an elliptic shape when the linearly polarized exposure light is applied to the photosensitive material through a photomask having a circular opening pattern. Therefore, in the case where a photomask having a circular opening pattern or the like is used, a circular pattern which is similar to the circular opening pattern in the exposure mask can be accurately formed on a photoresist material by placing the polarizer plate at the aforementioned second position outside the optical path and applying the unpolarized exposure light to the exposure mask.
(3) According to the second aspect of the present invention, there is provided a near-field exposure system comprising: a light source which emits exposure light having a predetermined wavelength and being linearly polarized; a phase plate which circularly or elliptically polarizes the exposure light; an exposure mask which has a pattern of openings each having a dimension smaller than the wavelength of the exposure light, and is placed at such a position that the exposure light is applied to the exposure mask; an exposure table which holds a photosensitive material sensitive to the exposure light, at a position which near-field light emerging from the openings reaches; and a phase-plate holding means which holds the phase plate in such a manner that the phase plate can be moved between first and second positions, where the first position is in an optical path of the exposure light from the light source to the exposure mask, and the second position is outside the optical path.
The near-field exposure system according to the second aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii).
(i) The near-field exposure system according to the second aspect of the present invention may further comprise an indication means for indicating a direction in which the exposure light emitted by the light source is linearly polarized.
(ii) The exposure light which is applied to the exposure mask may have a first component which is linearly polarized in a desired direction and a second component which is linearly polarized in a direction perpendicular to the desired direction, and the ratio of the second component to the first component may be 25% or smaller.
(iii) In the near-field exposure system having the above feature (ii), the ratio may be 15% or smaller.
(4) The advantages of the near-field exposure system according to the second aspect of the present invention are explained below.
The near-field exposure system as the second aspect of the present invention comprises a phase-plate holding means, so that the phase plate which circularly or elliptically polarizes the linearly polarized exposure light can be moved between the first position in the optical path of the exposure light from the light source to the exposure mask and the second position outside the optical path. That is, the phase plate can be selectively inserted in the optical path of the exposure light to the exposure mask.
In the case where the exposure mask has an opening pattern constituted by only lines extending in an identical direction, a fine pattern constituted by lines having small widths can be formed on a photosensitive material by placing the phase plate outside the optical path, achieving coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization, and applying the linearly polarized exposure light to the exposure mask.
The coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization can be achieved, for example, by changing the direction of the exposure table on which the photosensitive material is held, or adjusting the direction of the photosensitive material held on the exposure table.
In addition, in the case where the indication means for indicating a direction in which the exposure light emitted by the light source is linearly polarized is provided, the user can visually confirm the direction of the linear polarization in the operation for achieving the coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization. Therefore, the operation for achieving the coincidence between the above directions becomes further easier.
Further, in the case where a photomask having a circular opening pattern is used, a circular pattern which is similar to the circular opening pattern in the exposure mask can be accurately formed on a photoresist material by inserting the phase plate in the optical path of the exposure light to the exposure mask and applying the circularly or elliptically polarized exposure light to the exposure mask.
(5) The advantages of the near-field exposure systems according to the first and second aspects of the present invention having the additional features indicated in paragraphs (1) (iii), (1) (iv), (3) (ii), and (3)(iii) are explained below.
When the ratio of the component of the exposure light which reaches the exposure mask and is linearly polarized in a desired direction (i.e., the component linearly polarized in the direction identical to the direction of the lines constituting the exposure mask) to the component of the exposure light which reaches the exposure mask and is linearly polarized in a direction perpendicular to the desired direction is 25% or smaller, thickening of lines formed by exposure (i.e., the ratio of the increase over the mask pattern line width to the mask pattern line width) can be suppressed to 50% or less. When the thickening of lines formed on a photosensitive material based on a mask pattern having a line-and-space ratio of 1:1 is 50% or less, adjacent lines formed on the photosensitive material do not overlap.
Further, when the ratio of the component of the exposure light which reaches the exposure mask and is linearly polarized in a desired direction to the component of the exposure light which reaches the exposure mask and is linearly polarized in a direction perpendicular to the desired direction is 15% or smaller, the thickening of lines formed by exposure can be suppressed to 30% or less. It is said that the thickening of 30% or less is preferable for obtaining a very fine pattern constituted by lines formed by exposure.